1. Field of the Invention
The present invention relates to generally to continuous-time filters for signal processing, and, in particular, to a method and system for tuning precision continuous-time filters.
2. Description of the Related Art
Continuous-time filters are commonly used in communications systems, especially high frequency communication systems and magnetic storage read channels. One possible implementation of a high frequency continuous-time filter is a transconductance-capacitance (“Gm-C”) filter. A Gm-C filter consists of a transconductance (Gm) element and a capacitor (C). In this example, the transconductance element is characterized by the equation Iout=Gm*Vin, where lout is the output current, Vin is the input voltage, and Gm is the transconductance or gain of the element. This output current lout is applied to capacitor C to produce an output voltage. The voltage across capacitor C varies in accordance with the current through the capacitor, and the current through capacitor C varies in accordance with the voltage applied to the transconductor, thus creating a frequency dependent filter.
One type of Gm-C filter is a Gm-C biquadratic (“Gm-C biquad”) filter. A Gm-C biquad filter is a second-order recursive linear filter, meaning that its transfer function is the ratio of two quadratic functions and, thus, has two poles and two zeros. Higher-order recursive filters may be implemented using serially cascaded Gm-C biquad filters. Gm-C biquad filters are commonly used in tunable continuous-time band-pass filter applications. An example of such a Gm-C biquad filter can be found in the paper by Uwe Stehr & Frank Henkel, et al., A FULLY DIFFERENTIAL CMOS INTEGRATED 4TH ORDER RECONFIGURABLE GM-C LOWPASS FILTER FOR MOBILE COMMUNICATION, Proceedings of the 2003 10th IEEE International Conference on Electronics, Circuits and Systems, Vol. 1, pp. 144-147, (Dec. 14-17, 2003), which is incorporated herein by reference in its entirety.
The parameters of a Gm-C filter can vary with process, voltage and temperature (“PVT”) conditions. For instance, the value of Gm is dependent upon operating temperature, process variations such as transistor doping levels, and production variations such as transistor channel width, transistor channel length, etc. Thus, in applications requiring high accuracy, the Gm-C filter must be tuned to take the PVT variations into account to maintain proper cutoff frequency and quality factor of the filter. Existing methods of maintaining accuracy across PVT variations include manual tuning methods, analog tuning loops, and phase/edge rate detection. However, these existing methods can be cumbersome and may not maintain high accuracy.